The loss of hearing, and all that entails, has devastating consequences for human speech and language both in children and adults. The discovery of hair cell regeneration in birds has provided new hope of eventually restoring hearing in humans through hair cell regeneration. For this reason, it is important to know whether hearing is fully restored following hair cell regeneration in birds and, if not, what are the permanent, enduring effects on hearing and vocal production. Progress in this area has been challenging because, until recently, there is a complete absence of animal models for studying the effect of hearing loss on vocal output and vocal learning. The present research plan continues effort to understand the recovery of function following hair cell regeneration. Now we focus specifically on two species that provide quite different but unique opportunities for understanding the recovery of function. This new, more focused effort is the result of new and exciting findings from the last project period. First, we focus on understanding the relation between hearing recovery in budgerigars and the recovery in vocal precision and vocal learning. A new method for controlling vocal production and learning in an animal opens a whole new window on the study of hearing loss and vocal production. The second focus is on the Belgian Waterslager Canary. Our previous work confirmed that this species has an inherited hearing disorder due to missing or damaged hair cells on the basilar papilla, that they are constantly regenerating new hair cells, and that increasing the rate of regeneration can "cure" some of the inherited threshold shift observed in BWS canaries. [unreadable] [unreadable]